1. Field of the Invention
The present invention relates to a developing apparatus for use with an image forming apparatus, which visualizes a static latent image formed on a latent image bearing member using toner particles, which is a coloring material.
2. Description of Related Art
Such an image forming apparatus that has adopted an electrophotographic method used for copying machines, printers, etc. is provided with a developing apparatus composed so as to form a static latent image on the surface of a photosensitive member, which is a latent image bearing member, then supply a developing material such as toner particles (a coloring material) to the photosensitive material so that the toner particles are adhered thereon selectively to visualize the latent image.
The toner image obtained by developing the static latent image formed on the photosensitive member is transferred onto a transfer material composed of a sheet, etc. After this, part of toner particles that are not used for a transfer process remains on the surface of the photosensitive member. This unnecessary residual toner particles are removed from the surface of the photosensitive member to make the subsequent image forming to be performed repetitively. Thus, the developing apparatus is provided with a cleaning device for removing such residual toner particles from the surface of the photosensitive member after the transfer process is ended. The unnecessary residual toner particles removed by the cleaning device are then collected in a toner container provided in the cleaning device.
As the image forming apparatus provided with such a developing apparatus as described above is downsized, the space for installing an image forming device around the photosensitive member. Thus, such the recent trend toward smaller size of the image forming apparatus necessarily requires a smaller developing apparatus strongly.
Especially, a developing apparatus as described above, provided with a magnetic brush type developing roller for carrying a developing material composed of toner and magnetic carrier particles to a developing area corresponding to the photosensitive member using a magnetic force, collects residual developer particles in the developing tank after a developing process is ended. For this purpose, the collected developer is separated completely from the developing roller once in order to stabilize the developing, then new developer particles stirred and mixed enough are supplied to the developing roller. Consequently, the developing process for visualizing a latent image is stabilized, thereby high quality visual images (toner images) can be obtained.
In the prior art, after a developing process is performed by the developing roller, the tip of a scraper is put in contact with the surface of the developing roller so that collected developer particles are separated forcibly from the developing roller without fail. In such a developer separating mechanism, however, since the tip of the scraper is pressed against the developing roller, toner particles are fused and stuck on the surface of the developing roller, causing problems, for example, rising of the potential and lowering of the developer feeding force. The developing state of images is thus affected by those factors significantly, resulting in disabling of stable developing.
To avoid such troubles, therefore, an odd number of magnetic poles are composed in a magnet provided in a non-magnetic cylindrical developing sleeve composing the developing roller and those magnetic poles are disposed so that adjacent magnetic poles have the same polarity in an area where developer particles are separated from the developing roller after a developing process is ended. Consequently, developer particles are freed from the magnetic force and separated naturally from the developing roller. This is the mainstream for avoiding the above problems.
For example, as shown in FIG. 7, a developing apparatus 72 used to visualize a static latent image formed on a photosensitive member 71 is disposed so as to be opposed to the photosensitive member 71, which is a latent image bearing member. The developing apparatus 72 is provided with a developing roller 74 rotationally so as to be opposed to, especially, the opening of a developing tank 73 containing a developer composed of toner and carrier. The developing roller 74 is exposed partially at the opening of the developing tank 73 so as to be disposed at a distance from the photosensitive member 71.
The developing roller 74 is composed so as to provide a magnet 74b in a non-magnetic cylindrical developing sleeve 74a. The magnet 74b, as illustrated, is composed of an odd number of magnetic poles; a developing pole N1 corresponding to the developing area; a feeding pole S1 and a collecting pole S3 in adjacent to each other and with the developing pole N1 therebetween; a magnetic force distribution adjusting pole N2 in adjacent to those magnetic poles S1 and S3; a developer separating pole N4; an auxiliary feeding pole S2 adjacent to the pole N4; and a sucking pole N3 for sucking a developer onto the surface of the developing roller. In such a magnet 74b, 7 poles are provided. The magnetic force becomes almost zero at the magnetic poles N3 and N4 where the polarity of N3 and N4 becomes the same as that of respective adjacent poles. Thus, the developer is separated naturally from the developing roller 74.
Corresponding to the developer separating area described above, a stirring & feeding roller 75 is disposed. The roller 75 formed like a screw is used to stir and supply a developer. This stirring & feeding roller 75 stirs both toner and carrier particles fed to itself enough so as to be charged properly. The roller 75 supplies stirred developer particles to the developing roller 74. At this time, new developer particles are sucked onto the magnetic poles of the sucking pole N3 in the magnet 74b, so that the developer is fed to the next process.
The amount of the developer sucked and fed by the developing roller 74 is limited by a limiting member (doctor) 77 to a predetermined amount around the magnetic force adjusting pole N2. The developer limited and removed from the developing roller 74 is returned to the stirring & feeding roller 75 passing on top of a collecting plate 78. The developer is stirred and fed again to the developing roller 74. The developer passing the limiting member 77 is fed to the developing pole N1 via the feeding pole S1 positioned just before the developing pole N1, so that it is used for developing another latent image.
The developer rises like brush hair at the developing pole N1 in the magnetic force line direction and rubbed by the photosensitive member 71 thereby to develop a latent image. After this developing process, the developer is fed to the separating pole N4 via the collecting pole S3 and demagnetized and separated from the surface of the developing sleeve 74a. The developer is then stirred by the stirring & feeding roller 75 opposed to the developing sleeve 74a.
The magnetic pole direction, the magnetic force, and the number of poles of the magnet 74b forming a route for circulating and feeding the developer to the developing position, then to the separating position in the developing tank 73, are decided taking the system balance into consideration (especially taking a process speed into consideration). If the process speed is fast, sometimes the developer is fed to the developing area before the developer is not stirred and mixed enough. Consequently, the toner is used f or developing while the toner charging is insufficient and the toner density is not kept at a fixed value. As a result, image blushing and insufficient density problems are apt to arise.
In addition, if the stirring & feeding roller 75 is reduced in diameter and only one roller is used (usually, two rollers are used) to cope with a smaller apparatus, the developer is supplied to the developing roller 74 before the supplied toner is not stirred and mixed enough in the longitudinal direction of the stirring &feeding roller 75. As a result, image blushing and insufficient density will occur more often.
In order to solve such problems, generally, the shapes of the screw and the blades of the stirring & feeding roller 75 are contrived. For example, it is already known that if the tip of a roller blade comes in contact with the developer stuck on the developing roller by its magnetic force, the amount of charged developer particles is increased at the portion immediately, thereby it appears as density nonuniformity in developing a half tone image. For example, as shown in FIG. 11, when a toner image is transferred onto paper after a developing process, stripes 79, each different in density from others, appear in the half tone image.
There is another countermeasure to prevent the above problems; the rotation speed of the stirring & feeding roller 75 is increased. This countermeasure, however, applies much stress to the developer. Thus, the rotation speed of the roller 75 must be decided taking various balances into consideration. And, even when the above balances are examined fully, it was difficult to obtain stable developing characteristics, since the developer condensing property was changed due to the ambient conditions including temperature, humidity, and heat generated by the speeded-up rotation of the stirring & feeding roller 75.
Furthermore, a developer cannot be replaced satisfactorily on the outer periphery surface of the developing roller 74 if the prior art magnetic poles of the magnet 74b are disposed as shown in FIG. 7. When the toner particles are moved from the developing roller 74 to a static latent image in the developing area, therefore, consumption of the toner on the surface of the developer is started at the outermost portion. Thus, many ghosts (ghost; a phenomenon causing a toner image to be whitened or blackened, since a pattern depending on an amount of toner consumed in the developing area appears as an image memory in the next process) occur. In order to make the developing apparatus compact in size, therefore, the developing apparatus is confronted with many problems. And, finally, the configuration of the developing apparatus will become complicated.
In the case of a developing apparatus that uses a two-component developer, the positions of the magnetic poles and the magnetic force distribution of the magnet are changed delicately, since the magnetic poles of the internal magnet affect each other. This is why it is not easy to distribute the magnetic force to the function of each portion on the developing roller. The relationship between the function of each portion of the developing roller and the function of the whole magnet is not cleared yet.
The magnetic force of each of the magnetic poles is varied according to not only the distribution of the magnetic force of the magnet, but also according to the magnetism of carrier particles and the amount of the carrier sucked on the surface of the developing roller when the carrier particles included in the developer lie therebetween. In addition, according to the speed of the sleeve rotating independently of the magnet provided in the developing roller, the moving or transition state of the magnetic force is varied significantly. This is well known through experiences. The mechanism is not disclosed yet, however.
Japanese Examined Utility Model JP-Y2 3-50536 (1991) has disclosed that if th e magnet incorporated in the developing sleeve of the developing roller composing the developing apparatus described above is composed so that a flat magnetic pole is provided at a site corresponding to each function and the magnetic force and the magnetic flux distribution are defined, then the stress applied to the developer is reduced while the developer (a mixture of toner and carrier) is supplied to the developing sleeve, as well as the risen portion of the developer is limited, and the developer is separated from the developing sleeve and collected into the developing tank.
Furthermore, the official report of Japanese Examined Patent Publication JP-B2 6-93153 (1994) has disclosed a method for preventing scattering and falling of toner and carrier particles in the developing area by setting a composite magnetic field vector (a magnetic field component vector obtained by compounding the magnetic field component in the radial direction and the magnetic field component in the tangential direction) larger than the peak value of the magnetic field component in the radial direction on the sleeve of the developing pole around the developing pole of the magnet and setting the range within 45 degrees from the center of the developing pole.
Furthermore, Japanese Patent Number. JP-B2 2505800 has disclosed a method for stabilizing the packing state of both toner and carrier particles by easing the magnetic density distribution curve at the downstream side more than at the upstream side in the feeding direction of the developer with respect to the developer supply magnetic pole distribution.
In recent years, as processings in a copying machine and a printer are getting faster and faster, technologies for securing a certain developing performance in the developing area are also required for a developing apparatus. One of those technologies is to make the outer diameter of the developing roller larger and larger thereby to increase the nip more in width (a range for a developer to come in contact with in a developing process) between the photosensitive member and the developing roller in the developing area. Because of this enlargement, the number of magnetic poles of the fixed magnet provided in the sleeve composing the developing roller is also increased. And, when the number of magnetic poles of the fixed magnet is increased, it also becomes complicated to decide each of parameters of the magnetic pole direction, the magnetic force, and the number of magnetic poles. In order to improve this, therefore, it is needed to study the developer and the mechanism of the function of the entire magnet. Thus, the disposing of the magnetic poles and the magnetic force configuration of the magnet in the developing apparatus described in the above Patent Publications cannot cope with the above problems any longer.
Hereunder, therefore, the problems of the prior art developing apparatus shown in FIG. 7 will be described in detail.
In the developing apparatus shown in FIG. 7, how the image quality is degraded according to a change of toner density will be described with reference to FIG. 8. In FIG. 8, the developing tank 73 is not illustrated. At first, how a developer flows in the developing apparatus will be described.
(1) Toner particles supplied from a toner supply part (for example, a toner cartridge) are stirred and mixed with carrier particles in a developer by the stirring & feeding roller 75. This is initial charging of the toner.
(2) After the initial charging, the developer is sucked by the sucking pole N3 of the fixed magnet 74b provided in the non-magnetic developing sleeve 74a. At this time, the developer is rubbed strongly between the sucking pole N3 and the stirring & feeding roller 75 as shown in FIG. 9, thereby the developer is charged more highly at the stirring & feeding roller 75 than at the developing roller 74 (charged more at the tip of each risen portion of the developer).
(3) Toner particles in the developer are rubbed against the stirring & feeding roller 75 so as to be charged together with carrier particles, then fed to the doctor 77 according to the rotation of the developing sleeve 74b after a process (2) is ended. A verification example for how the developer is fed will be described later. As shown in FIGS. 2A to 2C, when the developer is f ed to the next pole at the downstream side in the rotational direction of the developing sleeve sequentially, the developer is rotated and moved repetitively. Each risen portion of the developer i s assumed as a unit at this time.
(4) The interval between the developing sleeve 74a and the doctor 77 is decided to be a predetermined thickness experimentally. The N2 pole and the S1 pole are disposed in order from the upstream side in the rotational direction of the developing sleeve 74a with the doctor 77 therebetween, so that a fixed amount of developer is held on the developing sleeve 74a.
(5) The developer having passed the doctor 77 is fed into the developing area at a proper thickness. Then, the developer is rotated thereby to develop a static latent image on the photosensitive member accordingly. After this developing, the developer is collected into the developing tank 73.
(6) The developer having returned into the developing tank is separated from the surface of the developing sleeve 74a between the magnetic poles N4 and N3 of the magnet 74b. The developer is then stirred again by the stirring & feeding roller 75 thereby to adjust the amount of toner particles to be stuck on the surface of each of carrier particles to a fixed amount. Thus, the process (1) is repeated.
In the processes (1) and (2), when the developer passes the pole N1 once, the toner in the developer is moved toward the photosensitive member 71 according to a static latent image formed on the photosensitive member 71. In this case, the toner movement is started at the risen portion of the developer, closer to the photosensitive member 71. If the gap between the photosensitive member 71 and the developing sleeve 74a is smaller than the risen portion of the developer, the tip of the developer looks broken while in contact with the developing sleeve. After leaving the developing area, however, the risen portion of the developer on the surface of the developing sleeve 74a changes its height and amount while it is moved. Consequently, the state of the developer 80 becomes as follows after leaving the developing area; the toner density becomes low at the photosensitive member 71, that is, at a portion away from the surface of the developing sleeve 74a and high at the surface of the developing sleeve 74a as shown in FIG. 8.
In the above state, the developer is fed to the stirring & feeding roller 75 according to the rotation of the developing sleeve 74a. When the developer reaches the collecting pole S3, the developer in which much toner is stuck on each of carrier particles is stuck more at a portion away from the surface of the developing sleeve 74a. In addition, when the developer is fed toward the stirring roller, the developer in which much toner is stuck on each of carrier particles is found more on the surface of the developing sleeve 74a and the developer is separated from the surface of the developing sleeve 74a between the separating pole N4 and the sucking pole N3. New developer particles are supplied at the stirring & feeding roller 75, repeating the processes (1) to (6) described above. Thus, the developer is circuited and fed in the developing apparatus.
At this time, as shown in FIG. 8, part of toner particles 79 are stuck on the surface of the developing roller 74a between the separating pole N4 and the sucking pole N3, thereby a thin skin layer is formed with the stuck toner particles 79.
In addition, in the case of the prior art, the toner particles in the developer 80 are charged much more at the stirring & feeding roller 75 (away from the outer periphery surface of the developing sleeve), since the developer is rubbed against the stirring & feeding roller 75 at the developer sucking pole N3 as shown in FIG. 9. The developer in this state is rotated and moved to the next pole repetitively as shown in FIGS. 2A to 2C. Each risen portion is assumed as a unit at this time.
When the developer 80 reaches the developing pole N1, the developer in which toner particles are charged more comes to the photosensitive member 71 (away from the surface of the developing sleeve 74a).
Consequently, if the magnetic poles are disposed just like in the prior art magnet shown in FIG. 7, then the toner particles charged more partially after they are rubbed against the stirring & feeding roller 75 are used for developing in the developing area. This will then arise a problem that nonuniformity and blushing appear in images remarkably. This is also true in the developing apparatus described in each of the above Patent Publications.
Furthermore, if the collecting pole S3 is provided around the top (closest to the guide) of the guide 73a of the developing tank 73, the pressure for making the developer come in contact with the guide 73a is increased, thereby the toner is apt to scatter and blow out more frequently.
If the developer is not supplied enough to the sucking pole N3 opposed to the stirring & feeding roller 75, it causes a developing nonuniformity corresponding to the rotation of the blade of the stirring & feeding roller 75 (having a width corresponding to the thickness of the blade of the roller) thereby to reduce the uniformity of the half-tone density. Thus, stripe-like meandering uneven portions appear in images as shown in FIG. 11 as developing nonuniformity to appear in the prior art developing apparatus.
It was found that such a phenomenon was apt to occur when only one stirring & feeding roller was used to simplify a high speed machine with fast process speed and a stirring structure.
Furthermore, if the interval between the separating pole N4 and the sucking pole N3 is wide, a composite magnetic field component (repulsive magnetic force) 81 appears on the surface of the developing sleeve 74a between the poles N4 and N3, which has only a weak magnetic force originally as shown in FIG. 10. Thus, the developer is not separated completely from the surface of the developing sleeve 74a and accordingly the developer is not replaced completely with new one. This causes the image nonuniformity and the toner scattering.